A MEMS microphone measures the instantaneous differential pressure between opposing sides of diaphragm. To sense an acoustic wave impinging on a diaphragm, the static pressure on both sides of the moving diaphragm should, ideally, be the same.
In a typical MEMS microphone, springs suspending the diaphragm are patterned in a slot adjacent to the diaphragm (e.g., a gap between the diaphragm and a support structure surrounding the diaphragm), and configured to have the desired spring constant. Portions of the slot not occupied by the patterned springs inherently provide air leakage channels, which allow ambient air to balance the pressure of the diaphragm at both sides. Such channels represent an acoustic short circuit. Indeed, if their impedance is too small, most of the air flows through them and only a fraction of the acoustic energy in the air is imparted to the diaphragm to make the diaphragm move, thereby reducing the sensitivity of the microphone. The impedance of these channels is represented by a resistance, which considers losses due to viscous resistance of air passing through them, and an inductor, which represents the inertial effect of the air mass in the channels. At low frequency the resistance is dominant. So the low frequency response of the microphone is limited by the design of the spring channel slot geometry. Below the low corner frequency, the magnitude response has a significant decay.
The acoustic resistance of the slots (Rslot) is a function of the viscosity of the ambient air (ηair), and the thickness (tslot), length (Lslot) and width (wslot), and may be expressed as the flowing equation:
      R    slot    =      12    ⁢          η              j        air              ⁢                  t        slot                              L          slot                ⁢                  w          slot          3                    As such, the acoustic resistance of the slot is proportional to its thickness, and inversely proportional to the length of the slot and the cube of the width of the slot. In a spring based microphone design, the length of the spring slots are hundreds of microns and the width of the slot is about 1 micron. The low corner frequency Flow_corner is calculated by:
      F    low_corner    =      1          2      ⁢      π      ⁢                          ⁢                        R          slot                ⁡                  (                                    C              diaph                        +                          C              cavity                                )                    where Cdiaph and Ccavity are the acoustic compliance of diaphragm and package cavity respectively.
As such, lowering a microphones low corner frequency may depend on increasing the acoustic resistance, which in turn depends on controlling the dimensions of the slot. As such, the acoustic performance of prior art MEMS microphones is determined, in part, by limits on the ability of the microphone's fabrication process to make very small slots. In other words, the width and length of the slots (wslot) is determined at least in part by the limits of photolithography and etching methods available for use in fabricating the microphones.